It has heretofore been proposed to form the cover of a pressurized vessel and a nuclear reactor from a plurality of cover sections which are assembled and are tensioned together by tractive or tensioning arrangements extending through passages in the cover sections.
The cover sections can be composed of concrete, steel-reinforced concrete, cast iron or cast steel, but preferably are composed of sections of cast steel.
The cover can be mounted upon the cylindrical shell of the vessel by tension cables extending through the cover, the cables running parallel to the axis or generatrices of the cylindrical vessel and serving in part as the axial pretensioning means of the cylindrical portion.
In the conventional construction of the cover, cover sections are sectors of a circle lying in a single layer and held in coplanarity by shear resisting keys or wedges which take up the shear forces between adjoining cover sections. The keys have a rectangular cross section orthogonal to the key axes and the transfer of shear forces is effected in only one dimension (transverse to the axis), resulting in the application of high edge forces to the keys and the cover sections. When efforts to prestress such covers radially are made, it is found that it is impossible to obtain a predetermined or prestress in central regions of the cover and, regardless of how the single layer body forming the cover is constructed, there is a tendency for the central portion of the latter to belly vertically and render the prestress ineffective.